Lamp cap construction

ABSTRACT

A high-temperature lamp having seals which may be prone to failure as a result of oxidation at elevated temperatures has a lamp cap with a sleeve fashioned from expanded metal which surrounds the seal regions, the expanded metal promoting heat dissipation from the seal regions, primarily by radiation and convection, so as to keep the seals below temperatures at which oxidation is a problem.

ii iStiates .iiatent [1 1 London, England [22] Filed: 7 Apr. 4, 1973 [21] Appl, No.: 347,773

[30] Foreign Application Priority Data May 3, 1972 Great Britain 2'0671/72 [52] US. Cl 313/43, 313/44, 313/45 [51] Int. Cl. H01] 61/52. [58] Field of Search 313/43, 44, 45 [56] I References Cited UNITED STATES PATENTS 3,089,972 5/1963 Larson eta]; ..313/43' ohinson .lul 23, 1974 Y LAMP CAP CONSTRUQTION Primary Examiner-John Kominski [75] Inventor gfi g gg Buckley Robinson London Assistant ExaminerDarwin R. l-lostetter g Attorney, Agent, or FirmRobert F. OConnell; Dike, [73] Assignee: Thorn Electrical Industries Limited, Br nst in, Roberts & Cushman 57 ABS CT A- high-temperature lamp having "seals which may be' I from the seal regions, primarily by radiation and convection, so as to keep the seals below temperatures at which oxidation is a problem.

10 Claims, 2 Drawing Figures PATENTEU 3.825.785

SHEEI 2 BF 2 FIGZ an improved lamp capconstructionQThe invention is particularly, although no'texclusively, applicable to lamps, such as certain tungsten-halogen lamps, which operate at high temperatures. 7

In hightemperature constructions-,wherefused silica or quartz envelopes are chosen, molybdenum'is commonly used for lead-in conductors to the internal components, for example discharge electrodes and filaments, of electrondischarge devices and lamps. The molybdenum is usually in the form'of a thin foil which is sealed into the material forming the lamp envelope. Although molybdenum and silica or quartz have different coefficients of thermal expansion a strong atomic surface bond between the metal and silica is obtained. Allied with the flexibility of the molybdenum foil, the bond results in a good hermetic seal.

A' drawback inherent in the use of molybdenum is its tendency to oxidise at relatively low temperatures, which leads to premature, randomlamp failures. Oxidation begins when-the seal temperature exceeds about 350-450 C and becomes increasingly troublesome as the temperature increases. Failure of lamps may occur because oxidation results in loss of electrical conduc-' tivity through a lead-in conductor incorporating the ,foil. Alternatively, there is a progressive volume increase during oxidation which can cause the adjoining envelope material to crack, thereby destroying the seal and allowing ingress of air into the'envelope.

Premature failures are particularly common amongst conventional high wattage studio lamps, for example tungsten halogen lamps'of KW or more. The filaments of such lamps may still have many hundreds of hours of life when the lamps fail through breakdown .of

It has been found that a conventional tungsten halogen studio lamp of SKW' output has an operating envelope temperature of 800. The temperature of the seal region thereof is 450550 C. By applying the present invention to a similar lamp, the temperature of the seal region can be reduced to 350450 C. The rate of oxidation of the molybdenum lead-in conductor foils is'accordingly reduced to an almost non-existent level.

. Lamplife is thus substantially extended and variation of life from lamp to lamp is minimised.

The temperature within the cap is low enough for reflectorplates fabricated from polished aluminium to retain their polish. Costly noble metal reflectors are therefore unnecessary and overall cost of the lamp is kept relatively low.

The invention will now be described by way of examples with reference to the accompanying drawing wherein FIG. 1 shows a tungsten halogen studio lamp with its lamp cap cut-away for clarity and FIG. 2 shows a double-ended lamp using the invention.

The studio lamp has a tubular quartz envelope or bulb'll hermetically sealed and cotaining, inter alia, a

the seal. Attempts to reduce the rate of oxidation by v coating the bases of their envelopes with reflecting deposits or by providing internal baffles in their envelopes have not proved successful in overcoming'oxidation' failures. External surface baffles are costly because noble metals wouldbe needed otherwise oxidation would occur. Deposited coatings would also be unsatisfactory since'adherence would be poor in view of the high operating temperature of the lamp envelopes.

'The object of the present invention is to provide a lamp or discharge device whose terminal cap aids in dissipation of heat away from the seal region to prolong the useful life of the lamp or device." I I According to the present invention, there is provided a lamp or discharge device including an envelope containing a filament or discharge electrodes connected to external terminals via lead-in conductors which pass through the envelope and are sealedthereto, the or each seal region of the envelope being contained within a hollow cap having a perforated metal wall extending about the seal region. Preferablythe ratioof the area ofmetalinthe said wall to thetotal area of the perfora-.

should preferably be located between the actual seal.

region and the adjacent partof the'envelope whilst being spaced fromithe latter t of theshell23remote from the lamp envelope which gaseous halogen filling. The envelope 11 has two tubular extensions 12 at its lower end which terminate in press or pinch seals 13. A pair of conductive support wires 14, 15 extend from the seals 13 the lengthof the envelope 11 and carry two insulating bridges 16. The bridges in turn carry filament supports 17 between which a filament 18 is disposed, the two ends of the filament l8 being electrically connected to the two wires '14 and 15 respectively. I

Each wire 14, 15 protrudes into a seal 13 and is electrically connected to one end of a molybdenum foil 19 which is embedded-in the seal 13 and 0001-0004 inch thick. The other end of the foil iselectrically connected to a lead-out wire 20 which is, in turn, .coupled to one of a'pair of terminal pins 21 held in an insulating terminal block 22,

" The terminal block 22 is secured to the envelope l1 metal area'ratio of about 50 percent. The expanded, metal shell 23 is fastened to the terminal block 22 by crimping or cementing. Fasteners in the form of metal connector strips 26 grip the seals 13 and secure the shell 23 to the envelope 1 1, the strips 26 serving to conduct or heat sink heat from the seals 13 to'the shell An aluminum reflector plate 25 is contained within and supported by the shell 23, the plate having a polished'surfacefacingthe lower end of the envelope 11 The polished plate 25. is positioned between the-seals 13 and the adjacent end of the envelope 11, and functions to reflectjsome of theheat radiation entering the interior of the shell 23 back into. the envelope. In this way, some of the heat emitted by the lamp 10 is prevented from reaching the region within the lamp cap where the seals 13 are situated. This helps to reduce the temperature within that region. It is found that the plate 25 retains its polish throughout the life of the The expanded metal shell 23 is particularly effective in minimising the temperature within the lamp cap. The thin strands of metal results in the shell 23 having a relativelypoor heat conductivity down the shell 23. Parts may have an operating temperature of about 900 C are, in comparison, quite cool, and thus act as an efficient heat sink for the seal through the mounting strips 26. This, in conjunction with the large overall aperture area, permits a very efficient flow of cooling air to enter and leave the interior of the shell 23. Also, transfer of heat radiation away from the seals 13 to the shell 23 for dissipation is promoted. In use, it is found that the seal temperatures are of the order of 350-450 C when the envelope is at 800 C, so that the effect of oxidation of the molybdenum foils 19 is rendered negligible.

Whilst expanded metal is best in dissipating heat, other perforated metals such as meshes could be used. However, a high aperture/metal area ratio is necessary, otherwise little or no reduction in seal temperature would occur. If this area ratio were too low, the shell would have a high heat conductivity. As a result the metal, heated by both conduction from the envelope 1 l and heat radiated from the seals 13 could have a higher temperature than the ambient air temperature within the shell. The temperature of the air actually within such apertures as are present would tend to take up a temperature equal to the surrounding metal. Accordingly, there would effectively be no thermal gradient through the apertures and the passage of cooling air would be inhibited.

The present invention is primarily intended to be used in conjunction with high temperature tungsten halogen lamps. However, the improved cap structure is also of use in other lamps and devices where it is desired to improve cooling for example to minimise damage to lamp fitments and where the lamp itself is fragile when hot.

It has been found that the reflector plate 25 does not have to be polished aluminium. An unpolished ceramic plate can be used instead as an efficient reflector of infra red radiation.

In the described embodiment, the lead-in conductors pass through separate, adjacent seals and the two seal regions are contained within a single cap. Clearly, other single-ended lamps may utilise a single seal region for the lead-in conductors. The cap therefore contains only one seal region, a construction which may be used at each end of a double-ended" lamp 27, as shown in FIG. 2. In a double-ended lamp, the two caps 28 are substantially identical to that described above.

It will be recognised that the invention is applicable to both double and single ended lamp devices.

1 claim:

1. An electric lamp including: an hermetically sealed envelope, terminals for connecting said lamp to an electrical supply, lead-in conductors for transmitting electrical energy from said terminals to the interior of said envelope, said envelope having at least one seal region through which said conductors enter said envelope, and a hollow cap fitted to said envelope, said cap having a heat-dissipating wall which extends about said seal region, said wall being a perforated metal member.

2. A device according to claim 1, wherein the ratio of the area of metal in said wall to the total area of the perforations therein is of the order of 50 percent.

3. A device according to claim 2, wherein said wall is made from expanded metal.

4. A device according to claim 1, wherein a fastener secures said cap to said associated seal region of said envelope, said fastener being metallic to serve to conduct heat from said seal region to said cap.

5. A device according to claim 1, wherein a reflector is mounted within the interior of said cap to reflect heat radiation entering the said cap from said envelope back into said envelope.

6. A device according to claim 5, wherein said reflector is located between the actual seal region and the adjacent part of said envelope, and is spaced from the latter.

7. A device according to claim 5, wherein said reflector is a polished aluminium plate.

8. A device according to claim 1, wherein said envelope has two separate, adjacent seal regions and both are contained within the same lamp cap.

9. A device according to claim 1, wherein said envelope is an elongated tube whose opposite endseach include a seal region, said two seal regions being enclosed within substantially identical lamp caps with perforated metal walls.

10. A device according to claim 1 in the form of a tungsten halogen lamp, wherein said envelope is made from quartz or silica, and each of said conductors includes a molybdenum foil embedded in said envelope material, thereby forming an hermetic seal. 

1. An electric lamp including: an hermetically sealed envelope, terminals for connecting said lamp to an electrical supply, leadin conductors for transmitting electrical energy from said terminals to the interior of said envelope, said envelope having at least one seal region through which said conductors enter said envelope, and a hollow cap fitted to said envelope, said cap having a heat-dissipating wall which extends about said seal region, said wall being a perforated metal member.
 2. A device according to claim 1, wherein the ratio of the area of metal in said wall to the total area of the perforations therein is of the order of 50 percent.
 3. A device according to claim 2, wherein said wall is made from expanded metal.
 4. A device according to claim 1, wherein a fastener secures said cap to said associated seal region of said envelope, said fastener being metallic to serve to conduct heat from said seal region to said cap.
 5. A device according to claim 1, wherein a reflector is mounted within the interior of said cap to reflect heat radiation entering the said cap from said envelope back into said envelope.
 6. A device according to claim 5, wherein said reflector is located between the actual seal region and the adjacent part of said envelope, and is spaced from the latter.
 7. A device according to claim 5, wherein said reflector is a polished aluminium plate.
 8. A device according to claim 1, wherein said envelope has two separate, adjacent seal regions and both are contained within the same lamp cap.
 9. A device according to claim 1, Wherein said envelope is an elongated tube whose opposite ends each include a seal region, said two seal regions being enclosed within substantially identical lamp caps with perforated metal walls.
 10. A device according to claim 1 in the form of a tungsten halogen lamp, wherein said envelope is made from quartz or silica, and each of said conductors includes a molybdenum foil embedded in said envelope material, thereby forming an hermetic seal. 